In the art of reflection seismology, acoustic pulses are generated at the upper regions of a sea or an ocean, and reflected acoustic signals are measured and analysed. This technology is useful, for instance, for mapping the ocean floor and for exploring for oil and gas, in which case the structure below the floor surface is to be mapped.
The acoustic waves travel in the water as pressure waves, and are detected by pressure sensors. In a practical setup, a large plurality of sensors is arranged along the length of a cable of several kilometers long, with a mutual distance in the order of a few meters. The cable, indicated as “streamer”, is towed in the water behind a ship. Measuring signals from the sensors travel along the streamer to a processing apparatus, usually located aboard the ship. In practice, the ship will be towing a plurality of such streamers parallel to each other, at a mutual distance in the order of about 50 meters. So all in all, a measuring array of many thousands of pressure sensors will be in operation.
The detection of acoustic waves is also utilized in sensors that are placed on the ocean bottom, either as single-spot sensors (Ocean Bottom Node) or as a series of sensors arranged in a cable. Further, the detection of acoustic waves is not limited to the use in exploration but is more broadly utilized in seismic detection, i.e. the detection of seismic waves, including such waves that may result from earthquakes.
In a typical prior art example, the pressure sensor is implemented as a piezo element, which comprises a piezo crystal. Pressure variations cause the piezo crystal to contract or expand, which in turn causes the piezo crystal to generate electrical signals. In such case, for transporting these electrical signals, a streamer needs to contain electrically conductive lines, which are typically made of copper, but which may alternatively be made of aluminium. In order to keep signal losses low, the conductive lines must be relatively thick. Alternatively, or additionally, such streamers need to include data acquisition units for combining and multiplexing or digitising the sensor signals. The same applies to other types of streamers, where the pressure sensors generate electrical signals.
It has already been proposed to replace the electrical signals by optical signals. This would allow the copper signal lines to be replaced by optical fibers. Instead of active sensors, which themselves generate optical signals, passive sensors have been proposed. With the phrase “passive” in this context is meant that an optical property of such sensor varies in response to variations in an ambient parameter, which optical property can be measured by interrogating the sensor with light. A passive optical element that has proven itself in this respect is a so-called Fiber Bragg Grating (FBG) reflector.
An FBG reflector consists of an optical fiber wherein, at some location, a series of material modifications is arranged lengthwise in the fiber. Normally, the optical properties of an optical fiber are constant along the length, which optical properties include the refractive index. Such material modification, however, has a slightly different refractive index. A plurality of such material modifications, at mutually the same distance, behaves as a grating, which typically is reflective for a small wavelength band. If a light pulse is made to enter the fiber, substantially all wavelengths will pass the grating location but light within said small wavelength band will be reflected. At the input end of the fiber, a reflected light pulse will be received, of which the wavelength is indicative for the mutual distance between the successive material modifications.
Such FBG reflector sensor is typically sensitive to (local) strain. Variations in strain cause variations in length of the fiber, including variations in distance between the successive material modifications of the Bragg grating. These distance variations, in turn, translate to variations in the wavelength of the reflected light.
It is noted that FBG reflectors are known per se, and that the use of FBG reflectors in streamers is known per se. Reference in this respect is made to, for instance, US patent applications 2011/0096624 and 2012/0069703, both of which are incorporated herein for all purposes. Since the examples of the present invention described herein are not directed to providing an improved optical fiber or an improved FBG and since the present invention can be implemented using optical fibers with FBG reflectors of the same type as currently are being deployed, a more detailed explanation of design and manufacture of optical fibers with FBG reflectors is omitted here.
In situations when the acoustic waves to be sensed are pressure waves in the sea water, since the FBG reflectors are mainly sensitive to longitudinal strain variations, a pressure sensor having an FBG reflector as sensitive element needs to have means for translating pressure variations to fiber strain variations.
At least some examples of a pressure sensor device useful according to the present invention have an FBG element as sensing element, that is suitable to be used for measuring water pressure waves in a streamer for use in marine surveying and exploration. It is to be noted, however, that such pressure sensor device may also be useful in other applications.
In some examples, (e.g., an application in a streamer or other type of cable), the pressure sensor device should have a cross section as small as possible, preferably less than a few cm. For a good measuring result, the pressure sensor device should be as sensitive as possible to the acoustic pressure signal, i.e. pressure variations within a frequency range of 0.5 Hz to several tens of kHz, it being noted that the frequency range of interest depends on the actual application. On the other hand, a streamer may be used close to the sea surface but also at a depth of for instance 40 m or more. Other applications for the sensor will require a usability at substantially larger depths, up to ocean bottom depth, typically 3000 m. Therefore, the pressure sensor device should be sensitive to very small pressure variations superimposed on a static background pressure that may vary in a range from 0 to perhaps 300 bar(g). Further, depending on the application, the pressure sensor device should preferably have low sensitivity to disturbances as caused by, for instance, flowing water.
It would be advantageous if the pressure sensor device were robust. In some examples, the sensors may be arranged in devices that should operate properly without the need for maintenance or repair over time periods of many months, and/or devices that are “handled” more often. Further, ideally, in the transport process from manufacturer to final destination, the pressure sensor device should be capable to withstand temperature in range from about −60° C. to about +70° C.
Further, the pressure sensor device should be small. Application in a cable, for instance a streamer, means that there is only limited space available to the pressure sensor device, and this applies particularly to the cross section. US patent application 2004/0184352, U.S. Pat. No. 6,882,595, incorporated herein by reference for all purposes, discloses a design where a fiber is wound tight on a hollow mandrel, wherein pressure variations cause variations in the mandrel diameter and consequently in the fiber length, but such design has several drawbacks. One drawback relates to the fact that winding the fiber obviously makes it necessary to bend the fiber. However, the radius of curvature of the bend should not be lower than a certain minimum, which puts a minimum to the diameter of the mandrel, which in turn translates to a relatively large diameter of the cable. For a streamer, it is however desirable to reduce the diameter as much as possible, because that would result in less material, less weight, less drag, and lower operational costs. Also, if cables are wound, a larger diameter is a disadvantage. Further, in the design of said US 2004/0184352, the operation relies on changing the length of the fiber between FBG sensing elements, due to excitation with acoustic waves. But the length of the fiber also changes due to mechanically induced excitations, due to variations of strain in the cable. Depending on application, yet especially in the case of streamers, the strain in the cable varies because of “jerk” stresses and swell waves. This causes background noise. It would be preferred if the sensor were less sensitive, ideally insensitive, to length variations of the fiber between FBG sensing elements.
Further, in the design of said US 2004/0184352, the operation relies on the fact that, when a hollow mandrel is subjected to an increase of outside pressure, its internal volume decreases in proportion to the pressure increase. The optimum response is achieved if the axial length of the mandrel does not change, but even then the response of the change in circumference, which is proportional to the change in length of the fiber, is only proportional to the square root of the change in pressure.
In the case of a design where a fiber is wound tight on a hollow mandrel, such as disclosed in said US 2004/0184352, this would mean that the FBG sensing element would be located in the fiber portion that is wound on the mandrel, which is a bent fiber portion. It is however not desirable to have the FBG sensing element in a bent fiber portion, because best accuracy is achieved when the FBG sensing element is subjected to axial tension only.
A pressure wave in a fluid can be considered as a dynamic pressure signal on a static pressure background. As will be explained in the following, it is desirable for a pressure wave sensor to be insensitive to changes in the static background pressure.
FIG. 1A is a graph illustrating schematically the wavelength response spectrum of an FBG. It is noted that the wavelength spectrum of a fiber laser would look similar, qualitatively. The horizontal axis represents wavelength, the vertical axis represents signal magnitude (arbitrary units). As already mentioned above, the FBG typically is reflective for a small wavelength band centered around a basic response wavelength λR. For sake of clarity, the width of this band is exaggerated in the figure.
Assume in FIG. 1A that the basic response wavelength λR as shown applies for the case of atmospheric ambient pressure. In the case of traditional pressure sensors based on FBGs, pressure variations will translate to length variations and hence to variations of the response wavelength λR. Thus, the position of the response wavelength λR can vary over a response range RR, wherein the width of this response range RR depends on the minimum and maximum values of the pressure to be expected, and also on the ratio between pressure variations and wavelength shifts (response factor, amplification factor).
In a practical situation, a single fiber may contain multiple FBG sensor portions along its length. For distinguishing the various reflection signals originating from the different sensors, if no time domain multiplexing is used, the different sensors are set to have mutually different basic response wavelengths λR. This can for instance be done by having mutually different grating parameters of the various FBGs and/or by giving the respective fiber portions mutually different bias tension. The setting of the different sensors will be such that the corresponding response ranges do not overlap. FIG. 1B is a graph comparable to FIG. 1A, on a different scale, schematically showing five adjacent response ranges with their corresponding wavelength response spectrums. It is to be noted that in practice the number of sensors N may be smaller or larger than five.
For practical purposes, only a small overall bandwidth is available for the sensors. While the precise location of this bandwidth may depend on the fiber composition, a suitable example is a range from 1510 nm to 1550 nm, i.e. a bandwidth of 40 nm. Under static circumstances, this entire bandwidth would be available for the N sensors, and each sensor could have a response range of 40/N nm wide.
However, especially in the case of sensors to be used under water, it is a problem that the ambient pressure is not constant. If a sensor is to be used at depths from 0 to 40 m, the ambient pressure varies over 400 kPa. This is a pressure range much larger than the pressure variations expected due to acoustic waves. For absolute pressure sensors, i.e. pressure sensors that respond to the absolute pressure, the response wavelength λR would be shifted over a large distance. Assuming a shift of 50 fm/Pa as a reasonable approximation, a shift of the response wavelengths of 20 nm is to be expected. This would mean that only 20 nm bandwidth would actually remain available for the sensors, i.e. each sensor could only have a response range of 20/N nm wide.
This is illustrated in FIG. 1C, which is a graph comparable to FIG. 1B, on a different scale, schematically showing the five adjacent response ranges with their corresponding wavelength response spectrums at low pressure (close to the surface, indicated at A) and at high pressure (deep in the water, indicated at B), assuming that higher pressure results in longer wavelength. This sensitivity to ambient pressure means that the response range per sensor, and hence the dynamic pressure range that each sensor can handle, decreases with increasing water depth range, and/or that the possible number of sensors per fiber decreases. The latter option is hardly feasible in the case of streamers having a fixed number of sensors per fiber, especially when used at depths much deeper than 40 m.
It is to be noted that a similar problem exists for sensors where higher pressure results in shorter wavelength.
It is further to be noted that an FBG sensing element can be used in several configurations, which have in common that they comprise an FBG portion. If the FBG sensor is to be used for interrogation by external light, while the FBG portion reflects light of which the wavelength matches the grating, such sensor will be indicated as “reflector”. It is also possible that the FBG sensor is used as mirror portion in a fiber laser, so that the laser output wavelength matches the grating; the laser may for instance be a distributed feed back (DFB) fiber laser, or a distributed Bragg reflector (DBR) fiber laser. It is noted that the wavelength spectrum of a fiber laser looks similar, qualitatively, to the single wavelength reflection spectrum of FIG. 1A.
It is further noted that the fiber can be single core or multi core.
In the above, a problem has been described that relates to an FBG sensing element. It is noted, however, that the present invention is not only related to problems with FBG sensing elements. Any sensor type will give a “zero signal” if the variable to be sensed is zero, and will give an actual signal within a dynamic range corresponding to the dynamic range of the variable. If the variable is varied in a relative narrow range at a relative large distance from zero, the same will hold for the actual measuring signal, which in general will imply a low signal to noise ratio, if the sensor is sensitive to the absolute value of the variable, It is therefore more generally desirable to have a pressure sensor device in which the sensitivity to the static pressure is very small or even zero, so that the dynamic range of the sensor output signal is closer to the “zero signal”.